1. Field of the Invention
The present invention relates to circuits for controlling a multipulse rectifier by means of ignition pulses. More particularly, the invention relates to such circuits in which the timing between the start of each ignition pulse used for controlling the main valves of the rectifier is precisely regulated.
2. Discussion of the Prior Art
In known control units for rectifiers, a periodic reference voltage generated by an oscillator synchronized by a clocking AC voltage, such as a triangular voltage, is compared with a DC control voltage (DIN 41750 of February 1972, Sheet 7, Page 4, FIG. 2). The points of intersection of the periodic reference voltage and the DC control voltage determine the starting points of the firing control pulses and, hence, of the fired pulses. The oscillator frequency is usually chosen so that two ignition pulses, mutually staggered by 180.degree. (electrical), are formed in one period and fed to the control path of the associated controlled main valves of the rectifier. Therefore, only one oscillator is required for a two-pulse rectifier. Where control units for higher-pulse rectifiers are involved, a number of comparisons between phase-shifted periodic reference voltages of several oscillators and the DC control voltage is made. For example, three such oscillators, generating reference voltages which are phase-shifted by 60.degree., may be provided for a six-pulse recitifier. The oscillators can be adjusted "soft" so that they are able to follow dynamic control processes which can cause phase jumps in the clocking AC voltage.
It is a requirement that, for the prevention of asymmetries in the output voltage of the recitifier, the time intervals between the starts of the ignition pulses for the controlled main valves of the recitifier be equi-distant as accurately as possible. This means that, for high accuracy requirements, the oscillators and comparators must be balanced against each other very precisely and must show only minor changes in response due to the effect of temperature change or due to aging.
It is also possible to provide a control unit for a higher-pulse rectifier which has only a single oscillator and which generates a periodic reference voltage of a correspondingly higher frequency for comparison with the DC control voltage. The ignition pulses derived therefrom are switched cyclically to the controlled recitifier main valves. In steady-state operation, good firing angle symmetry can be achieved with such a circuit, provided the stability of the oscillator is sufficient. However, such a "hard"-adjusted oscillator is then unable to follow dynamic control processes associated with phase jumps in the clocking AC voltage.
It is an object of the present invention to provide an improved control unit for a multi-pulse rectifier having an improved firing angle symmetry.